1. Field of the Invention
The present invention relates to a hydraulic suspension system for a vehicle such as an automobile or the like, and more particularly relates to an improved hydraulic suspension system in which an abrupt change in vehicle height is prevented when the system is started.
2. Prior Art
Conventional hydraulic suspension systems or vehicle height control systems for vehicles have, in general, a plurality of hydropneumatic actuators each being provided corresponding to vehicle wheels and adapted to increase and decrease the vehicle height as respective results of the supply and the discharge of working fluid to and from its working fluid chamber, working fluid supply passages for supplying working fluid to the working fluid chambers, working fluid discharge passages for discharging working fluid from the fluid chambers, connecting passages communicated with the working fluid chambers, and control valves each being adapted to selectively connect the associated connecting passage with the working fluid supply passage or the working fluid discharge passage so as to control the supply and the discharge of the working fluid to and from the working fluid chambers.
As described, for example, in Japanese Utility Model Laying Open Publication No. 62-202404 (1987) and Japanese Patent Laying Open Publication No. 63-106132 (1988), in such conventional hydraulic suspension systems or vehicle height control systems, it has already been known in the art to incorporate an on-off valve in each connecting passage so as to prevent the vehicle height from decreasing due to the undesirable phenomena that some of the working fluid flows out of the working fluid chamber during such a period when a pump is not operated which serves to supply high pressure working fluid to the working fluid supply passage or in such an event where any trouble has happened in the working fluid discharge passages or the like. The on-off valves are kept in their closed positions while the system is not operated and are opened when the vehicle height is adjusted or while the system is operated.
In the hydraulic suspension systems or vehicle height control systems having such on-off valves incorporated, the undesired decrease in vehicle height is effectively prevented. In such systems, however, if the on-off valves are opened under the condition where relatively large pressure differences exist across them when the system is started, a relatively large volume of working fluid will flow through each on-off valve at the instant when it is opened, which causes temporarily to change the vehicle height abruptly so that passengers might feel queer.
The problem is specifically more noticeable in the system wherein, as shown in FIGS. 8 and 9 in Japanese Patent Application No. Showa 63-284278 which was filed by an applicant the same as one of the assignees of the present application and which it is not intended hereby to admit as prior art to the present application except to the extent in any case required by applicable law, the control valves are pressure control valves each being adapted to regulate the pressure within the associated working fluid chamber and the on-off valves are cut-off valves each being adapted to respond to the pressure within the associated working fluid supply passage upstream of the pressure control valve and to remain in its closed position whenever the pressure is not more than a predeterminate value. Namely, in the last mentioned systems, since the pressure within the working fluid supply passage upstream of the pressure control valve increases from a level which is lower than the predeterminate value when the system is started to operate and each cut-off valve opens when the pressure within the supply passage has exceeded the predeterminate value, we can not accurately know when the cut-off valve opens, and therefore can not know the controlling pressure of the pressure control valve to be set at the time point when the associated cut-off valve opens.
As is also shown in FIG. 7, in the above mentioned Japanese Patent Application No. Showa 63-284278 the on-off valves may be cut-off valves which are provided on the opposite side from the associated actuator relative to the pressure control valve and are adapted to remain in their closed positions whenever the pressures within the associated working fluid supply passages upstream of the cut-off valves are not more than a predeterminate value. In such a system, although no problem will occur during the period before the cut-off valves open even if the instructed controlling pressures of the pressure control valves are zero or far different from the actual pressures within the working fluid chambers, the pressure control valves will act when the on-off valves have been opened so that working fluid flows relatively rapidly through the pressure control valves, causing a temporary abrupt change in vehicle height.